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Showing papers on "Palladium published in 2001"


Journal ArticleDOI
21 Sep 2001-Science
TL;DR: Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires fabricated by electrodeposition onto graphite surfaces and transferred onto a cyanoacrylate film.
Abstract: Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires. These palladium "mesowire" arrays were prepared by electrodeposition onto graphite surfaces and were transferred onto a cyanoacrylate film. Exposure to hydrogen gas caused a rapid (less than 75 milliseconds) reversible decrease in the resistance of the array that correlated with the hydrogen concentration over a range from 2 to 10%. The sensor response appears to involve the closing of nanoscopic gaps or "break junctions" in wires caused by the dilation of palladium grains undergoing hydrogen absorption. Wire arrays in which all wires possessed nanoscopic gaps reverted to open circuits in the absence of hydrogen gas.

1,315 citations


Journal ArticleDOI
TL;DR: Mild synthetic protocols for the formation of aryl-carbon, aryL-nitrogen, and aryal-oxygen bonds based on soluble, well-defined copper(I) catalysts do not require the use of palladium and/or expensive ligands.

512 citations


Journal ArticleDOI
TL;DR: In this article, the authors discuss two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst.
Abstract: Membrane reactors applied to catalytic reactions are currently being studied in many places world-wide. Significant developments in membrane science and the vision of process intensification by multifunctional reactors have stimulated a lot of academic and industrial research, which is impressively demonstrated by more than 100 scientific papers on catalytic membrane reactors being published per year. Palladium as a noble metal with exceptional hydrogen permeation properties and, at the same time, broad applicability as a catalyst, first of all for hydrogenation, is part of many of these developments. This paper discusses two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst. Dense palladium composite membranes can be used for hydrogen separation from packed-bed catalysts in gas-phase hydrocarbon dehydrogenation reactions. Mesoporous membranes containing dispersed bimetallic Pd/X-clusters can be employed as so-called catalytic diffusers for liquid-phase hydrogenation, e.g. of nitrate and nitrite in water. The principles of both concepts are introduced, recently obtained experimental data are evaluated in connection with literature results, and the perspectives for further development are highlighted.

467 citations


Journal ArticleDOI
07 Sep 2001-Science
TL;DR: The electrochemical/catalytic properties of defined atomic ensembles in atomically flat PdAu(111) electrodes with variable surface stoichiometry that were prepared by controlled electrodeposition on Au(111), derived from infrared spectroscopic and voltammetric data obtained for electrode surfaces.
Abstract: Bimetallic electrodes are used in a number of electrochemical processes, but the role of particular arrangements of surface metal atoms (ensembles) has not been studied directly. We have evaluated the electrochemical/catalytic properties of defined atomic ensembles in atomically flat PdAu(111) electrodes with variable surface stoichiometry that were prepared by controlled electrodeposition on Au(111). These properties are derived from infrared spectroscopic and voltammetric data obtained for electrode surfaces for which the concentration and distribution of the respective metal atoms are determined in situ by atomic resolution scanning tunneling microscopy with chemical contrast. Palladium monomers are identified as the smallest ensemble ("critical ensemble") for carbon monoxide adsorption and oxidation, whereas hydrogen adsorption requires at least palladium dimers.

422 citations



Journal ArticleDOI
TL;DR: A review of the application of palladium metal catalysts to the Heck reaction can be found in this paper, where a brief outline of the historical development of heterogeneous catalysis as applied to catalysis is given.
Abstract: This review is devoted to the application of palladium metal catalysts to the Heck reaction (arylation or vinylation of alkenes with aryl or vinyl halides and pseudohalides). The number of relevant articles published in recent years shows steadily increasing interest in this field. A brief outline of the historical development of heterogeneous catalysis as applied to the Heck reaction is given (Section 2). Both supported metal catalysts (Section 3) and stabilized colloidal palladium catalysts (Section 4) are included. Heterogeneous catalysts supported over different kinds of supports (carbon, inorganic oxides, molecular sieves, polymeric materials, etc.) are reviewed under separate headings. Particular attention is paid to the metal leaching and the nature of catalysis, two tightly connected problems which appear to be still controversial (Section 5). Some perspectives are shortly discussed in the final section (Section 6).

377 citations


Journal ArticleDOI
TL;DR: In this article, the effect of co-metal on the performance of Pd in bimetallic catalysts is presented, and the main methods to prepare BMo catalysts through the unselective and selective depositions of the cometal precursor and the use of heterobinuclear complexes are highlighted.
Abstract: An overview about the effect of co-metal on the performance of Pd in bimetallic catalysts is presented. In the first part, the promotion of monometallic catalysts by addition of a second metal is described from a general point of view in terms of: (i) electronic effects; (ii) geometric effects; (iii) the occurrence of mixed sites; and (iv) the disappearance of the β-PdH phase. The second part presents the main methods to prepare bimetallic catalysts through the unselective and selective depositions of the co-metal precursor and the use of heterobinuclear complexes. The influence of preparation methods on the chemical state and the spatial distribution of both components are highlighted. In the last part, a short overview is given about the influence of co-metal addition on some important reactions in which Pd is the reference active phase: (i) selective hydrogenation of highly unsaturated aliphatic hydrocarbons; (ii) hydrogenation of aromatics or hydrodearomatisation; (iii) hydrogenation of nitrogen-containing compounds; (iv) reactions involving CO; and (v) hydrodechlorination.

361 citations


Journal ArticleDOI
Bin Xue1, Ping Chen1, Qi Hong1, Jianyi Lin1, K. L. Tan1 
TL;DR: By thermal decomposition of metal salts, palladium, platinum, silver and gold nanoparticles, with average size of 7nm, 8nm, 17-nm and 8-nm, respectively, were grown on carbon nanotubes as mentioned in this paper.
Abstract: By thermal decomposition of metal salts, palladium, platinum, silver and gold nanoparticles, with average size of 7 nm, 8 nm, 17 nm and 8 nm, respectively, were grown on carbon nanotubes. TEM observations showed that most of the nanoparticles synthesized stuck on the outer surface of the carbon nanotubes. The size of nanoparticles can be adjusted by changing the metal to carbon ratio. Small nanoparticles are prone to agglomerate under intense bombardment from the electron beam. By comparison, nanoparticles were scarcely found on graphite or active carbon using the same synthesis process. HRTEM and XRD investigations revealed the well-crystallized structure and a lattice contraction of these nanoparticles.

353 citations


Journal ArticleDOI
TL;DR: Heterogeneous catalysts prepared, characterized, and applied for the first time for catalytic carbon-carbon coupling reactions exhibit a high activity and selectivity toward the Heck reaction of aryl bromides with olefins for small palladium concentrations.
Abstract: [Pd]-exchanged NaY zeolites have been prepared, characterized, and applied for the first time for catalytic carbon-carbon coupling reactions. The catalysts exhibit a high activity and selectivity toward the Heck reaction of aryl bromides with olefins for small palladium concentrations (< or =0.1 mol % of Pd). The catalysts can easily be separated from the reaction mixture and reused after washing without loss in activity. No limitation to the diffusion of adducts in the zeolite cages was observed (for linear alkenes). The electronic nature of the aryl bromides and the olefins has a dominating effect on the reaction yield and selectivity. The heterogeneous catalysts quantitatively convert all types of all aryl bromide (complete conversion of bromobenzene within 30 min) and activated aryl chlorides under standard reaction conditions. Product form selectivity is observed in the Heck reaction with cyclic olefins.

331 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the synthesis of supported palladium catalysts with emphasis on a summary of the chemistry of catalyst synthesis, oxide and carbon support properties and case studies from the period 1990 to 2000.
Abstract: The synthesis of supported palladium catalysts is reviewed with emphasis on a summary of the chemistry of catalyst synthesis, oxide and carbon support properties and case studies from the period 1990–2000. With oxide supports aqueous precursor solutions are often used. The charge of the complex in combination with the iso-electric point (IEP) of the support is an important property to consider during synthesis affecting both dispersion and distribution over large support bodies. With carbon supports direct reduction of palladium precursor may occur that leads to large metallic Pd particles. With the latter support lyophilicity can largely affect Pd emplacement. With oxide and carbon supports sol–gel, deposition–precipitation, deposition–reduction, ion-exchange and impregnation methods have been applied. In general it turns out that the thermal treatment often dominates the primary application of the Pd-precursor in establishing the ultimate metal dispersion. Thermal treatment in an inert atmosphere at temperatures not exceeding 773 K prior to reduction is beneficial for the Pd dispersion. Gas-phase reduction leads to smaller Pd particles than does liquid-phase reduction. In our opinion large progress can be made to develop highly loaded thermostable Pd catalysts by utilizing deposition–precipitation techniques in combination with anchoring sites on the support.

314 citations


Journal ArticleDOI
TL;DR: In this paper, the experimental procedures for catalytic hydrodehalogenation are reviewed; the hydrogen source can be either molecular hydrogen or a hydrogen donor in the so-called hydrogen-transfer hydrogenolysis.
Abstract: Catalytic hydrodehalogenation is one of the most promising methods for detoxification of organohalogen waste. A variety of catalytic systems has been used, although palladium catalysts are the best choice because of their optimal catalytic properties. Catalytic hydrodehalogenation on palladium catalysts is carried out in both the liquid and gas-phases. The experimental procedures for the catalytic hydrodehalogenation are reviewed; the hydrogen source can be either molecular hydrogen or a hydrogen donor in the so called hydrogen-transfer hydrogenolysis. A wide variety of hydrogen donors has been used, including inorganic salts (phosphinates) organic salts (formates), alcohols, proaromatic compounds, etc. Kinetics and mechanisms proposed for hydrodehalogenation are presented, and both agreements and disagreements on the mechanistic aspects are discussed. Catalysts modification (base addition, support modification, second metal addition, etc.) as well as its transformation during the hydrogenolytic process (i.e. deactivation) are discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the activity for ammonia oxidation and the intermediates formed during the reaction and found that the metal electrodissolution is enhanced by ammonia under these conditions.

Journal ArticleDOI
TL;DR: In this article, the nitrate removal activity and selectivity for various pH values, and for different bimetallic catalysts, as well as results described in literature were used to set up a mechanistic model for the reaction.
Abstract: Technologies for the removal of nitrate from drinking water and waste water will be required in the near future, and the catalytic nitrate reduction is one of the most promising methods. So far, the understanding of nitrate reducing catalysts has been very poor. Experimental trends in nitrate removal activity and selectivity for various pH values, and for different bimetallic catalysts, as well as results described in literature were used to set up a mechanistic model for the reaction. The nitrate reduction activity is determined by bimetallic ensembles, at which nitrate is adsorbed and reduced to nitrite by hydrogen, which is supposed to spillover from palladium sites to the bimetallic sites. Formic acid, on the other hand, reacts with nitrate by a transfer hydrogenation mechanism from neighboring palladium sites. The extent of positive charging of the bimetallic sites is supposed to influence the activity at different pH values. At a high pH, strongly adsorbing oxygenated species block bimetallic nitrate adsorption sites as well as palladium sites. The selectivity is determined by the ratio of nitrogen species to reductant species at monometallic palladium sites. At these sites, the reduction of nitrite and other intermediates take place, finally leading to the end products. If this ratio of nitrogen to reductant species changes, the selectivity changes as well, e.g. at different ratios of the two metals. The trends in the experimental data are well described by this model.


Journal ArticleDOI
TL;DR: In this article, a number of functionalised heterocyclic carbene complexes have been synthesised from imidazolium salts with Ag2O, which act as carbene transfer agents when reacted with palladium salts.

Journal ArticleDOI
TL;DR: In this article, the major causes for deactivation and premature degradation of palladium catalysts are briefly summarised, including particle growth for various reasons, coke deposition and coke transformation, the influence of the support material on long term stability and modifications at the palladium surface itself such as valency changes or the formation of simple but stable molecular surface species.
Abstract: Some of the major causes for deactivation and premature degradation of palladium catalysts are briefly summarised. These include particle growth for various reasons, coke deposition and coke transformation, the influence of the support material on long term stability and modifications at the palladium surface itself such as valency changes or the formation of simple but stable molecular surface species. In addition, variations of physical properties as well as chemical poisoning, corrosion and leaching are discussed. In spite of the large body of literature concerning the common phenomenon catalyst deactivation specific information accessible for the various kinds of palladium catalysts being used worldwide is still quite limited. There is a serious need for future studies on properties of palladium-based catalytic systems regarding, for example, the interactions between carbon, hydrogen and supported palladium as a ternary system. Observations on different deactivation processes on Pd/SiO2 catalysts occuring under industrial conditions are compiled. It was tried to roughly differentiate between different degrees of coking and coke transformation in mainly thermally or purely catalytically driven catalyst coking on the one hand and of moderate or enhanced corrosion phenomena or changes of the properties of the palladium itself on the other. © 2001 Elsevier Science B.V. All rights reserved.



Journal ArticleDOI
TL;DR: In this paper, it was shown that air and thermally stable palladium(II) complexes of CNC and CCC bis-carbene pincer ligands have a twisted conformation and catalyze Heck olefination of activated aryl chlorides.

Journal ArticleDOI
TL;DR: A variety of mixed palladium(II) complexes bearing N-heterocyclic carbenes (NHCs) and triaryl-and trialkylphosphines [NHC(R2)]Pd(PR′3)I2 (R=Me, t-Bu, (R)-1-phenylethyl; R′=Ph, o-tolyl, cyclohexyl, T-Bu) have been prepared.


Journal ArticleDOI
TL;DR: In this article, a protocol for the synthesis of functionalized diaryl- and triarylamines under mild conditions, using a soluble, air-stable copper(I) complex, Cu(PPh3)3Br, as the catalyst and cesium carbonate as the base, was presented.

Journal ArticleDOI
TL;DR: In this paper, the peculiarities of the IR studies of adsorption on semiconducting oxides are reviewed with special attention to SnO2, and the specificity of SnO 2 is evidenced upon interaction of a stoichiometric (CO+NO) mixture at r.t.


Journal ArticleDOI
TL;DR: In this article, an alumina-supported palladium-copper catalysts, which promote liquid-phase nitrate reduction, have been prepared according to different impregnation sequences of γ-Al 2 O 3 support and characterized by X-ray photoelectron spectroscopy (XPS), Xray induced Auger electron spectroscopic (XAES), and temperature-programmed reduction (TPR).
Abstract: Alumina-supported palladium-copper catalysts, which promote liquid-phase nitrate reduction, have been prepared according to different impregnation sequences of γ-Al 2 O 3 support and characterized by X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (XAES) and temperature-programmed reduction (TPR). Analysis of Pd XPS/XAES spectra reveals that palladium is present on the γ-alumina support in the metallic form. Copper is reduced at lower temperatures in the presence of palladium particles compared to the CuO/γ-Al 2 O 3 sample. Due to the low copper content in catalysts ( σ Cu ≈10 14 atoms cm −2 ), the XPS/XEAS spectral features of reduced copper species are quite different from those of bulk copper. As shown by TPR and XPS/XAES data, formation of highly dispersed Pd-Cu bimetallic clusters is suggested. It was discovered by means of TPR analysis that the catalyst preparation in which the γ-alumina support is impregnated first by copper salt, results in higher formation of the Pd-Cu alloy.

Journal ArticleDOI
TL;DR: In this paper, the ceria-zirconia mixed oxide was used as an active catalyst for methane combustion, in the 673-1073 K temperature range, which is associated with an oxidized state of the catalysts, whereas a reduction at 573 K strongly activates the solids.


Journal ArticleDOI
TL;DR: In this article, the authors studied the catalytic properties of Pd supported on Al2O3, SiO2, TiO2 and ZrO2 for hydrogenation of carbon monoxide.
Abstract: Catalytic hydrogenation of carbon monoxide was studied over Pd supported on Al2O3, SiO2, TiO2, and ZrO2. Both the activity and the selectivity of the catalysts were strongly affected by the nature of the support. As a major product, Pd/Al2O3 produces dimethyl ether, Pd/SiO2 favors methanol formation, Pd/TiO2 produces CH4, and Pd/ZrO2 yields methanol. Higher CO conversions over Pd/ZrO2 and Pd/TiO2 were ascribed to the presence of cationic palladium species formed through the metal–support interaction. Significant dimethyl ether formation over Pd/Al2O3 was attributed to the acidity of the support metal oxide. The catalytic natures of Pd catalysts were discussed based on the results of catalyst characterization by XPS, TEM, and temperature-programmed reduction and desorption.

Journal ArticleDOI
TL;DR: In this article, the second-order rate constant for the reaction of chlorauric acid with pyrrole in dilute solution was found to he 13 M -1 s -1.
Abstract: Composite colloids of gold and polypyrrole were prepared using two different methods: 1, using pyrrole colloid, created by the oxidation of pyrrole by ferric chloride, to subsequently reduce chlorauric acid and, 2, oxidizing pyrrole monomer with chlorauric acid in a sodium dodecylbenzene sulfonate solution. In each case, the polypyrrole colloid consisted of irregularly shaped particles approximately 500 nm in diameter. The gold produced in each case was in the form of irregular spheres, approximately 407 nm in diameter in method 1 and 13 nm in method 2. X-ray photoelectron spectroscopy was used to determine the oxidation state of the species present. Transmission electron microscopy and light scattering data were used to determine the particle sizes of both gold and polypyrrole colloids. Energy dispersed spectrum X-ray analysis and electron diffraction were used to confirm the presence of metallic gold in the composite colloids. The second-order rate constant for the reaction of chlorauric acid with pyrrole in dilute solution was found to he 13 M -1 s -1 . Aqueous solutions of palladium, platinum, rhodium, cobalt, tin, silver, zinc, nickel, titanium, cadmium, mercury, arsenic, and selenium were also examined for their potential to act as oxidants to produce composite polypyrrole colloids. Palladium, platinum, and rhodium salts were suitable oxidants, producing polypyrrole in less than 12 h.